GASNet is a language-independent, low-level networking layer that provides network-independent, high-performance communication primitives tailored for implementing parallel global address space SPMD languages such as UPC, Titanium, and Co-Array Fortran. The interface is primarily intended as a compilation target and for use by runtime library writers (as opposed to end users), and the primary goals are high performance, interface portability, and expressiveness. The name, GASNet, stands for "Global-Address Space Networking".

The design of GASNet is partitioned into two layers to maximize porting ease without sacrificing performance: the lower level is a narrow but very general interface called the GASNet core API - the design is based heavily on Active Messages, and is implemented directly on top of each individual network architecture. The upper level is a wider and more expressive interface called the GASNet extended API, which provides high-level operations such as remote memory access and various collective operations.

We've written a network-independent reference implementation of the extended API purely in terms of the core API, which allows GASNet (and the GAS compiler) to quickly and easily be ported to a new network architecture by re-implementing only the minimal core API. GASNet is structured such that implementers can choose to additionally bypass certain functions in the reference implementation of the extended API and implement them directly on the hardware to improve performance of specific operations when hardware support is available (e.g. special network support for puts/gets or hardware-assisted broadcast).

Most clients will use calls to the extended API functions to implement the bulk of their communication work (thereby ensuring optimal performance across platforms). However the client is also permitted to use the core active message interface to implement non-trivial language-specific or compiler-specific communication operations which would not be appropriate in a language-independent API (e.g. implementing distributed language-level locks, distributed garbage collection, collective memory allocation, etc.). The AM features of the core provide a powerful extensibility mechanism which allows clients to implement a wide variety of specialized communication operations.